1. Field of the Invention
This invention relates to a canned motor pump pressurized recirculation system and, more particularly, to a canned motor pump pressurized recirculation system having an auxiliary pump to effect pressurization of the system.
2. Description of the Prior Art
In conventional canned motor pumps, some of the fluid which has been pumped is recirculated from the high pressure portion of the pump through the motor to the impeller in the low pressure or suction portion of the pump. This recirculation fluid absorbs heat from the motor which heat can result in vaporization or boiling of the fluid when it is returned to the low pressure portion of the pump. Vaporization of the fluid results in a vapor lock which causes the bearings of the motor to fail and the motor to burn out.
A known method to overcome such vaporization in the pump is "reverse circulation" wherein the recirculated fluid is directed from the discharge of the high pressure portion of the pump through the motor at a controlled temperature and pressure, and then is piped back to a low pressure or suction reservoir in a customer's system. However, the problem with reverse circulation is that the control conditions for each customer's system may vary and therefore no unifrom operational instructions can be employed. For instance, if customer's recirculation piping has low flow resistance, fluid in the motor will have a low back pressure so that it may boil. If the recirculation piping has high flow resistance, the amount of fluid flow is low and therefore the fluid in the motor will vaporize causing the motor to burn out.
The Hermetic Pump Company, Freiburg, Germany has devised a pump which attempts to overcome the problems of reverse circulation. In particular, fluid is taken from the discharge portion of a main impeller through the shaft of the pump to an auxiliary impeller which is located in the rotor portion of the cavity of the motor. The auxiliary impeller directs the recirculated fluid through the cavity of the motor back to the discharge portion of the main impeller. However, some of the disadvantages of the system are that the flow from the main impeller to the auxiliary impeller is limited by the size of the shaft of the pump, and alternatively if one wishes to reduce the flow this would be difficult since the path of the flow is located in the pump. Also, since the auxiliary impeller is inside the cavity for the rotor, the size of the auxiliary impeller is limited.